Metal-Oxide-Semiconductor (MOS) chemical sensors require operation at elevated temperatures (typically between 200 C and 500 C). Another common feature of these sensors is lack of sufficient gas discrimination. In some applications (e.g. fire detection) it is acceptable if not desirable to detect presence of several gases. For instance Chromium Titanium Oxide (CTO) MOS sensor is sensitive to several gases commonly found in fires. Presence of any of those gases would generate signal that can be used to detect fire event. However, CTO's sensitivity to alcohols and few other molecular compounds that are commonly found in normal environment is so high that their presence obfuscates detection of desired target gases.
There are different ways of dealing with this issue. One approach is to prevent offending gases to reach sensor surface. Another is to add coatings such as chrome oxide or catalytic material that modify sensitivity to different gases. Combination of several MOS sensors with different coatings can be used as an “electronic nose” with appropriate signal analysis. However, in some applications (e.g. portable gas sensors) power consumption is critical parameter and combination of several sensors can not be implemented. Another approach is to take measurement at different temperature settings or take measurements as the temperature is increasing followed by signal analysis. Rationale of this approach is that MOS sensitivity to each gas is a strong function of temperature and it is conceivable that deconvolution of this data can recover information about type of gas detected by MOS sensor. Complexity of this approach along with the need to operate at higher temperature makes this approach very difficult.
There is thus a continuing need for lower power consuming multi-sensor gas detectors where combination of several sensors increases selectivity of the gas sensor. In complex applications like fire detection there is a number of gases that can be generated in different amounts. It is beneficial if sensor can detect any of those target gases. However, MOS sensors are sensitive to gases that are found in common environments. To improve performance of a detector one can add one or more sensors with a high degree of gas selectivity that can:                Positively identify presence of one specific target gas        Positively identify presence of one or more offending gases, or        Do both.NDIR sensors have high degree of gas selectivity and although not perfect, can improve detection of events.        
Another issue with MOS sensor is its accuracy. Its sensitivity to gases is highly dependent on temperature at its surface. For that reason it is desirable to be able to maintain desired temperature level at constant value.